Averaged potentials evoked by presenting grating stimuli at a rate of 1 Hz were recorded from the human scalp, and compared with psychophysical contrast threshold for the same patterns. Effects of partial paralysis of eye muscles and of inhaling a mixture of N2O O2 (60:40 per cent) upon both the early and late components of the evoked potentials were found to be small. It was also confirmed that the late wave (P2) correlated with the pattern detection. The early component was confirmed to be correlated with the contrast threshold only for brief presentations of stimuli (40 msec). © 1973.